“But those who are careless of accuracy in small things soon begin to neglect the most important.”

1950s
Context: In matters concerning truth and justice there can be no distinction between big problems and small; for the general principles which determine the conduct of men are indivisible. Whoever is careless with truth in small matters cannot be trusted in important affairs.

(1955) as quoted in Albert Einstein: Historical and Cultural Perspectives (1997) ed. , p. 388, from The Centennial Symposium in Jerusalem (1979)

Address to his branch and flight control team on the Monday morning following the Apollo 1 disaster (30 January 1967), known as "The Kranz Dictum"; as published in Failure Is Not An Option : Mission Control from Mercury to Apollo 13 and Beyond (2000) by Gene Kranz, p. 204. The phrase "tough and competent" was echoed by NASA Director Sean O'Keefe following the Space Shuttle Columbia disaster, adding that "these words are the price of admission to the ranks of NASA, and we should adopt it that way."
Context: Spaceflight will never tolerate carelessness, incapacity, and neglect. Somewhere, somehow, we screwed up. It could have been in design, build, or test. Whatever it was, we should have caught it.
We were too gung ho about the schedule and we locked out all of the problems we saw each day in our work. Every element of the program was in trouble and so were we. The simulators were not working, Mission Control was behind in virtually every area, and the flight and test procedures changed daily. Nothing we did had any shelf life. Not one of us stood up and said, "Dammit, stop!"
I don't know what Thompson's committee will find as the cause, but I know what I find. We are the cause! We were not ready! We did not do our job. We were rolling the dice, hoping that things would come together by launch day, when in our hearts we knew it would take a miracle. We were pushing the schedule and betting that the Cape would slip before we did.
From this day forward, Flight Control will be known by two words: "Tough and Competent." Tough means we are forever accountable for what we do or what we fail to do. We will never again compromise our responsibilities. Every time we walk into Mission Control we will know what we stand for.
Competent means we will never take anything for granted. We will never be found short in our knowledge and in our skills. Mission Control will be perfect.
When you leave this meeting today you will go to your office and the first thing you will do there is to write "Tough and Competent" on your blackboards. It will never be erased. Each day when you enter the room these words will remind you of the price paid by Grissom, White, and Chaffee. These words are the price of admission to the ranks of Mission Control.

Vesicles make clouds; they are trifles light as air, but then they make drops, and drops make showers, rain makes torrents and rivers, and these can alter the face of a country, and even keep the ocean to its proper fulness and use. It teaches a continual comparison of the small and great, and that under differences almost approaching the infinite, for the small as often contains the great in principle, as the great does the small; and thus the mind becomes comprehensive. It teaches to deduce principles carefully, to hold them firmly, or to suspend the judgment, to discover and obey law, and by it to be bold in applying to the greatest what we know of the smallest. It teaches us first by tutors and books, to learn that which is already known to others, and then by the light and methods which belong to science to learn for ourselves and for others; so making a fruitful return to man in the future for that which we have obtained from the men of the past.
Lecture notes of 1858, quoted in The Life and Letters of Faraday (1870) by Bence Jones, Vol. 2, p. 403

1955) as quoted in Albert Einstein: Historical and Cultural Perspectives (1997) ed. Gerald Holton, Yehuda Elkana, p. 388, from The Centennial Symposium in Jerusalem (1979
1950s

Variant translation: Everything has a small beginning.
"De Finibus Bonorum et Malorum" Book V, Chapter 58

Source: On Love

As quoted in Peter's Quotations : Ideas for Our Time (1977) by Laurence J. Peter, p. 508; this begins with a phrase derived from one in the Tao Te Ching, by Laozi

319 U.S. 641
Judicial opinions, West Virginia State Board of Education v. Barnette (1943)